There are many applications for robust, accurate, low current and as always cost effective rotational sensor solutions.
In computer mouse applications, optical sensor solutions have been very popular and in many automotive applications magnetic (Hall) sensors have been used. A large number of these applications are digital by nature, for example one or multiple holes are positioned in a wheel (keyed) to determine the rotation and direction, and also speed if measured against time. FIG. 1 illustrates a prior art optical sensing solution for a computer mouse at 1. In a large number of prior art mice, as presented by 1.1, in addition to left 1.2 and right 1.3 buttons, a user may use a wheel 1.4 to navigate within an associated display. Typically, said wheel 1.4 revolves around an axle 1.7, and has an LED 1.8 and a photo diode or photo transistor 1.9 located on either side of it. A number of holes 1.6 is located around the wheel circumference, and allows infra-red or visible light to travel from the LED to the photo diode or transistor. As the wheel is rotated, light is alternately blocked or allowed to pass, with the resultant signal used to monitor rotation.
Hall sensors may be employed to monitor a magnetic field passing the sensor. This may be done with a single magnet passing once per revolution, or with multiple magnets to increase resolution. Another prior art implementation uses a single magnet with a plate attached to a wheel that generally shields the magnet from the Hall sensor, except at certain points around the wheel. This is effectively the same concept as that used by the optical solution. FIG. 2 illustrates these prior art concepts. At 2, a single magnet 2.2 is attached to a wheel 2.1 which rotates in direction 2.5 around an axle 2.3. A Hall sensor 2.4 is used to monitor the number of revolutions, with the Hall sensor output typically at a maximum when said magnet is adjacent to it. Such a solution can only determine rotational movement at a resolution of one turn, and can typically not be used to determine rotation direction. At 2.6, a wheel 2.7 which revolves in direction 2.8 around axle 2.9, and which absorbs magnetic fields to some extent, is shown. Wheel 2.7 is perforated with holes 2.12 about its circumference. A Hall sensor 2.10 and a permanent magnet 2.11 are aligned with each other, and located on either side of said wheel 2.7. When a hole moves between the Hall sensor and permanent magnet, more magnetic field lines from the magnet couples with the sensor, with an increase in its output. In this manner, wheel rotation may be monitored, with resolution dependent on the number of holes.
Construction and manufacturing tolerances can be a problem, and for Hall sensors the current (power) consumption is an inherent problem. If the wheel can be turned at 50 revolutions per second (3000 RPM), i.e. every revolution takes 20 milli-seconds (ms), and there are 200 holes (for magnetic field or light to pass through), the receiver must detect a pulse every 100 micro seconds (μs) in order to keep track of the movement, irrespective of the reporting rate required. This rather high speed sensing may result in high current consumption and the requirement for very accurate positioning of sensing system components.
As mentioned above, most computer mice today utilize some kind of rolling wheel which can be used to enter a scrolling command, whereby a cursor or other object displayed on a computer screen may be moved up or down based on the rotation of said wheel. Naturally, these computer mice comprise rotation sensing circuitry to monitor the movement of the mouse wheel. Typical mouse wheels are made out of plastic and weigh little, while their axles experience a fair amount of rotational friction. Consequently, when such a mouse wheel is rolled with a quick movement, it does not keep spinning, but rapidly comes to a stop. This makes it cumbersome to scroll a cursor over a large distance, for example through a lengthy document, as the mouse wheel has to be turned a large number of times. To overcome this, some higher end computer mice have wheel mountings with little rotational friction and a wheel having a fair amount of weight and therefore inertia. These allow users to perform a quick rolling movement to spin said wheel up, which then keeps spinning, resulting in continuous scrolling until the wheel is stopped by the user or till the wheel eventually stops by itself. However, due to the fine mechanical construction required, such a solution is not highly cost effective. The art may benefit from an invention that allows entry of a high speed, continuous scrolling command while using a low cost, low weight, high friction mouse wheel similar to what is presently employed in most mice, and which does not require the physical mouse wheel to keep spinning.